Ammonia is present in many wastewaters as a result of decomposition of nitrogenous waste products, such as urea and proteins, or as a result of protein metabolism by many aquatic animals, and can have harmful effects including increased eutrophication of lakes and depletion of dissolved oxygen. Ammonia is extremely toxic to aquatic animals and even at low levels can inhibit their growth. Reducing the concentration of ammonia to a harmlessly low level is therefore important for wastewaters which are to be returned to the environment, and it is essential for waters used to raise aquatic life. Closed systems for raising fish where the water is recycled especially need to include a water treatment step to remove ammonia since the fish density may be high and the ammonia will build up to toxic levels quickly in the absence of a method of removing it.
Four methods which have been used for removing ammonia from waters are (1) use of nitrifying bacteria to convert ammonia to nitrites and nitrates, referred to as biological filtration, (2) use of ion exchange materials to remove the ammonia from the water, (3) air stripping and (4) breakpoint chlorination. Of these methods, only the first two are useful for recirculating systems for aquatic life.
A biological filter may comprise an inert material, such as sand, gravel, plastics, and fibrous material, which serves as a substrate to hold bacteria which act on the ammonia in the water. Biological filters have limitations and their use can present problems since there are conditions under which the filter can be inoperative. The filter may be virtually inoperative upon start-up since it may take several weeks to establish a biological filter; low temperatures inhibit the nitrifying action; chemicals such as antibiotics used for disease control can kill the bacteria; the filter is not readily amenable to handling significant increases in ammonia concentration, as may occur upon addition of a large quantity of fish to a tank; and decrease in ammonia concentration below that needed for subsistence of the bacteria will cause bacteria to die and slough off, leaving the biological filter temporarily only partially effective in nitrifying ammonia.
The use of ion exchange resins presents different problems, primarily in the regeneration which is usually accomplished by exchange with chemical reagents. The regeneration of chemicals may be costly, and if neglected will result in ammonia break-through and the consequences following the elevated ammonia concentration.